Rom 6
10 For in that he died, he died unto sin once: but in that he liveth, he liveth unto God.
11
Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
12
Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.
13
Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God.
14 For sin shall not have dominion over you: for ye are not under the law, but under grace.
15 What then? shall we sin, because we are not under the law, but under grace? God forbid.
16 Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?
I have sinned even after I got saved. However, because I have died in Christ, I do not continue in my sin. I repented, God forgave me, and that sin is NOT counted against me. This is essentially JUSTIFICATION. God IMPUTES righteousness into me even though my actions do not deserve it.
Before I "died" I would continue in sin. After, I died, I no longer wanna sin. My desire is changed.
Notice, Paul, does NOT say, because we are dead in Christ, we cannot sin. In fact, he states quite the opposite.
Rom 6:12 Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.
If I let sin reign in my mortal body, I would obey its lust. But if I know that I am dead in Christ, then I should not allow sin to reign in me.
This is what Paul is saying
Fact: I died in Christ.
Faith: I believe that when Christ died, I died
Experience: Reckon myself to be dead in Christ and live according to the Spirit and not to the flesh anymore.
Consider this:
Col 3
1 If ye then be risen with Christ, seek those things which are above, where Christ sitteth on the right hand of God.
2 Set your affection on things above, not on things on the earth.
3
For ye are dead, and your life is hid with Christ in God.
4 When Christ, who is our life, shall appear, then shall ye also appear with him in glory.
5
Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry:
6 For which things' sake the wrath of God cometh on the children of disobedience:
7
In the which ye also walked some time, when ye lived in them.
8 But now ye also put off all these; anger, wrath, malice, blasphemy, filthy communication out of your mouth.
Paul is saying because I am dead in Christ, mortify my fleshly desires. So, if I don't mortify my fleshly desires (hence, allowing sin to reign in my mortal body), I will obey its lust. So yes, its possible to still sin as a believer, but we are not to live in sin by obeying its lust.